For memory devices and for numerous other applications, bistable devices or circuits are used. For example, for storing one bit of information in a memory, a bistable device can be used which is switchable between (at least) two different and persistent states. When writing a logical “1” into the device, it is driven into one of the two persistent states and when writing a logical “0”, or erasing the logical “1”, the device is driven into the other of the two different states. Each of the states persists until a next step of writing information into the device or erasing information in the device proceeds.
Flash erasable programmable read only memory (FEPROM or flash memory) is used in semiconductor devices and provides for rapid block erase operations. Flash memory generally requires only the use of one transistor per memory cell versus the two transistors per memory cell required for conventional electrically erasable programmable read only memory (EEPROM). Thus, flash memory takes up less space on a semiconductor device and is less expensive to produce. However, the need to develop further space saving components and to remain cost efficient in the fabrication of semiconductor devices continues.
To that end, the use of materials with bi-stable electrical resistance for semiconductor device applications has been studied. The resistance states of the material can be changed reversibly by applying short electrical pulses to the material. These pulses should be larger than a given threshold VT and longer than a given time t. The resistance state of the material can be read or analysed by applying other pulses which are non-destructive to the conductivity state if they are much smaller than VT.
U.S. Pat. No. 6,204,139 describes a method for switching properties of perovskite materials used in thin film resistors. The properties, in particular the resistance, are switched reversibly by short electrical pulses. Application of the method for memory cells and for sensors with changeable sensitivity is proposed.
The articles “Reproducible switching effect in thin oxide films for memory applications” by A. Beck et al., Applied Physics letters, Vol. 77, No. 1, July 2000, “Current-driven insulator-conductor transition and non-volatile memory in chromium-doped SrTiO.sub.3 single crystals” by Y. Watanabe et al., Applied Physics Letters, Vol. 78, No. 23, June 2001, and “Electrical current distribution across a metal-insulator-metal structure during bistable switching” by C. Rossel et al., Journal of Applied Physics, Vol. 90, No. 6, September 2001 and the international application publication WO 00/49659 A1 describe materials and classes of materials with programmable resistance, and simple resistor devices made from these materials.
Transition metal oxides are one class of materials that can be conditioned such that they exhibit the desired bi-stable electrical resistance. This conditioning process comprises subjecting the insulating dielectric material to an appropriate electrical signal for a sufficient period of time. This conditioning process generates a confined conductive region in the transition metal oxide that can be reversibly switched between two or more resistance states.
The confined conductive region is generated at an arbitrary position in the dielectric material, i.e., the position of the conducting path is not controlled by well defined process parameters. This leads to a large variation in the electrical properties of nominally identical memory cells comprising conventional programmable resistors and of devices comprising such memory cells.
The time-consuming conditioning process and the large variations of the properties of nominally identical programmable resistors used in the memory cells and of devices comprising such memory cells are severe drawbacks for manufacturability.